1. Field of the Invention
The present invention relates to a wire electric discharge machine having a function of compensating a machining current pulse of a wire electric discharge machine.
2. Description of the Related Art
The wire electric discharge machine machines a workpiece into a desired shape by applying an electric voltage between a wire electrode and the workpiece to generate electric discharge between them, while changing a relative position between the wire electrode and the workpiece. Generally, in the electric discharge machining of the workpiece, a required precision varies according to a material of the workpiece or a thickness of the workpiece, however, a precision in several microns order may be often required as a higher precision.
An example of a general wire electric discharge machine will be shown in FIG. 10.
A machining power source 3 includes a direct voltage source 21 and a switching element 22 which changes an electric voltage supplied from the direct voltage source 21 to a high frequency pulse. The generated high frequency pulse is fed to an electric supply portion 5 via an electric supply line 4, and is applied to a side of a wire electrode 1 which is supported by a wire electrode support portion 8, and a side of a workpiece 2.
Since the pulse applied to the side of the wire electrode 1 and the side of the workpiece 2 has a high frequency, the electric supply line 4 equivalently includes an inductance component. Further, the inductance component varies dependent upon the attitude of an electric supply line 4 and the like. Further, a distance from the electric supply portion 5 to an electric discharge point (that is, of an electric discharge route) is different according to the relative position between the wire electrode 1 and the workpiece 2, and a resistance component is differentiated by the difference. The way of flowing of electric current is differentiated by the change of the impedances of these electric discharge routes. There is a problem that the difference of the way of flowing of electric current directly affects a machining speed and a machining precision.
With regard to the problem that the way of flowing of electric current varies according to the relative position between the wire electrode and the workpiece, or the state of a subject to be machined, a technique of detecting a peak current and output voltage so as to compare the detected values with reference values, and adjusting the output voltage according to the result of comparison is disclosed in Japanese Patent Application Laid-Open No. 59-205228 or Japanese Patent No. 3820989. However, in these techniques, since the output voltage is adjusted by detecting the peak current and the output voltage or the like, and thereafter comparing the detected values with the reference values, a time delay is generated from the detection of the peak current or the output power to the reflection of the detected value on the adjustment of the output voltage. As a result, it is hard to perform a highly precise control so as to obtain a machining result having a high precision.
Further, Japanese Patent Application Laid-Open No. 10-138048 discloses a technique of measuring and compensating an average open voltage between an electrode and a workpiece in an open state in which a machining gap is spaced at a predetermined distance before carrying out the machining, and comparing the measured and compensated voltage with a previously stored reference voltage. However, this technique controls only such open voltage in the wire electric discharge machining, and does not compensate the electric current which actually contributes to the machining. Therefore, there is a case that a desired result of machining cannot be obtained in an actual machining.
Further, Japanese Patent Application Laid-Open No. 10-138048 discloses a technique of machining under a plurality of machining states, actually measuring machining information in each of the machining conditions, determining a machining characteristic of an electric discharge machine based on a machining condition and an actually-measured machining information in the machining condition, and compensating the machining condition and the machining information. However, the technique determines the machining characteristic of the electric discharge machine by actually measuring the machining information in the actual machining, and corrects the machining condition and the machining information based on the machining characteristic, however, the technique has a problem that it is impossible to compensate a state of the workpiece and an influence exerted by a jig.